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 Let us now glance at the fourth division—the ,—so called because of the disposition of the organs round a centre, which is the mouth. Our fresh-water vases afford us only one representative of this type—the Hydra, or fresh-water Polype, whose capture was recorded in the last chapter. Is it not strange that while all the Radiata are aquatic, not a single terrestrial representative having been discovered, only one should be found in fresh water? Think of the richness of the seas, with their hosts of Polypes, Actiniæ, Jelly-fish, Star-fishes, Sea-urchins, Sea-pens (Pennatulæ), Lily-stars (Comatulæ), and Sea-cucumbers (Holothuriæ), and then compare the poverty of rivers, lakes, and ponds, reduced to their single representative, the Hydra. The radiate structure may best be exhibited by this diagram of the nervous system of the Star-fish.

Fig. 19.

Cuvier, to whom we owe this classification of the animal kingdom into four great divisions, would have been the first to recognize the chaotic condition in which he left this last division, and would have acquiesced in the separation of the , which has since been made. This fifth division includes many of the microscopic animals known as Infusoria; and receives its name from the idea that these simplest of all animals represent, as it were, the beginnings of life.

But Cuvier's arrangement is open to a more serious objection. The state of science in his day excused the imperfection of classing the Infusoria and parasites under the Radiata; but it was owing, I conceive, to an unphilosophical view of morphology, that he placed the molluscs next to the Vertebrata, instead of placing the Articulata in that position. He was secretly determined by the desire to show that there are four very distinct types, or plans of structure, which cannot by any transitions be brought under one law of development. Lamarck and Geoffroy St. Hilaire maintained the idea of unity of composition throughout the animal kingdom;—in other words, that all the varieties of animal forms were produced by successive modifications: and several of the German naturalists maintained that the vertebrata in their embryonic stages passed through forms which were permanent in the lower animals. This idea Cuvier always opposed. He held that the four types were altogether distinct; and by his arrangement of them, their distinctness certainly appears much greater than would be the case on